The Lyme Disease Debate: Host Biodiversity and Human Disease Risk
Levy, Sharon, Environmental Health Perspectives
In the mid-1970s, several children in the town of Lyme, Connecticut, began to suffer from painful, swollen joints. They were diagnosed with juvenile rheumatoid arthritis, a rare condition. Clinicians at Yale School of Medicine suspected the cluster of cases was caused by an infectious agent. (1) The illness, dubbed Lyme disease, was soon recorded in an increasing number of patients in the northeastern United States. Symptoms included rashes, fevers, joint and muscle pain, and heart and neurological problems.
National Institutes of Health researcher Willy Burgdorfer identified the culprit in 1982: a spirochete bacterium that, in electron micrograph images, resembles a broken twist of barbed wire. (2) The spirochete, named Borrelia burgdorferi (Bb), was first isolated from the gut of ticks collected in woodlands on Shelter Island, New York, where Lyme disease had become endemic.
Thirty years after its discovery, Lyme disease has become by far the most common vectorborne infection in North America. (3) Today the ecology of Bb is the subject of both intense study and intense scientific debate. Some researchers think that protecting large tracts of forest habitat--a strategy that increases the diversity of vertebrate hosts for both Bb and its tick vectors--will ultimately reduce the risk of human infection. Others argue that there is no predictable relationship between host biodiversity and human disease risk.
History of Lyme Disease
Lyme disease occurs in Europe and Asia as well as North America, always spread by ticks in the genus Ixodes. (4) Over the last decade, about 20,000-30,000 U.S. cases of Lyme disease have been reported annually by the Centers for Disease Control and Prevention, the majority occurring in the Northeast and the Midwest, where the vector is the black-legged tick, Ixodes scapularis. (5) Average annual numbers of cases in Europe and Asia have been estimated at 65,467 and 3,450, respectively. (4)
The infection's sudden rise in the United States in the 1970s gave the impression that Lyme disease was caused by a newly invading pathogen, but the diaries of early American settlers reported abundant ticks, and the evidence now shows that Bb is an ancient infection in North America. Distinctive Bb genes have been identified in museum collections of ticks from the 1940s (6) and of white-footed mice from the turn of the twentieth century, (7) and studies of genetic variation in separate populations of Bb suggest the pathogen existed across much of the present-day United States many thousands of years before European settlement. (8) Nevertheless, genetic analyses indicate that this genus of bacteria originated in Europe. (9)
Bb is a microbe of forest habitats, and its history is tied to human land use. As European settlers moved west across the United States, they cleared great swaths of forest. Deer, one of the major hosts for black-legged ticks, were overhunted and dwindled to a few small, scattered populations. Populations of white-footed mice, an important reservoir host for Bb, also declined. But in some undisturbed spots in the Northeast and the Midwest, deer, white-footed mice, their tick parasites, and Bb all survived. With the abandonment of most northeastern farm fields in the mid-nineteenth century, forests regenerated, and the microbe traveled with its tick and vertebrate hosts into newly re-expanding habitats. (10)
Lyme disease now appears to be expanding outward from long-time refuges. (10) Migratory birds carry ticks to new habitats, enabling the spread of both ticks and Bb south and north. (11) Some bird species that host ticks are expanding their ranges north, and studies of emerging Lyme disease in Quebec, Canada, suggest that climate change makes it possible for tick vectors to survive in an area that once would have been too cold. (12)
Bb is hitched to the life cycle of its tick vector. Over the course of a life span that lasts at least two years, Ixodes ticks must take a blood meal from a vertebrate host on three separate occasions, dropping off the host after each meal. …